1. Field of the Invention
The present invention relates to a high frequency diode, a silicon wafer that has high resistivity and preferably used in the high frequency diode, and a method for producing the silicon wafer. Priority is claimed on Japanese Patent Application No. No. 2006-022831 filed Jan. 31, 2006, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, PIN diodes, TRAPPATT diodes, and IMPATT diodes have been known as devices having a high resistivity layer (I layer: Intrinsic semiconductor layer) interposed between a PN junction, and are used as devices for high frequency switching. The high-resistivity layer is a drift region of minority carrier, and a property related to the switching rate of the diode is controlled by lifetime that is a time in which the minority carrier flows in the high-resistivity layer.
Conventionally, a silicon wafer made of an FZ crystal produced by the floating zone (FZ) method, a substrate made of a CZ crystal produced by the Czochralski method (CZ method) or the like have been used as substrates for production of high-frequency diodes having a high-resistivity layer, for example, a PIN diode. By the FZ method, it is easy to form a crystal having high resistance. Where the CZ crystal is used, the substrate is produced by forming a high resistance epitaxial layer (<100 Ωcm) on the silicon wafer made of the CZ crystal. In the high-resistivity layer of the above-described substrate, recombination centers are formed, for example, by thermally diffusing heavy metals such as Au and Pt to the substrate, or by introducing irradiation defects by electron beam irradiation.
However, where a high-frequency diode is produced using a silicon wafer made of the FZ crystal, it is impossible to avoid the use of a crystal having a small diameter since it is difficult to produce a crystal having a large diameter by the FZ method. Therefore, using the FZ crystal, it is impossible to expect the enhancement of the productivity of the high-frequency diode.
On the other hand, where a high-frequency diode is produced using a silicon wafer made of a CZ crystal which may have a large diameter, there have been the following problems. Since a crystal growth of the CZ crystal is performed using a quartz crucible, the CZ crystal has high interstitial oxygen concentration. During a heat treatment at about 350° C. to 450° C. in a device production process, oxygen in the CZ crystal generates oxygen donors such as thermal donors (Old Donors) and New Donors. Therefore, it has been difficult to ensure a desired resistivity because of the fluctuation of resistivity before and after the heat treatment in the device production process.
Usually, since a substrate has a resistivity of 10 Ωcm or less, oxygen donors generated during the device heat treatment process only have negligible influence on the resistance of the substrate. However, in a substrate of P type crystal having high resistance, generation of oxygen donors during the device heat treatment process increase the resistivity. In addition, if the generation of the oxygen donors is further increased, the oxygen donors override the P type impurities and generate an inversion from P type to N type resulting in a decrease of resistivity. Such phenomena remarkably fluctuates the resistivity. A method for producing a CZ crystal having low interstitial silicon concentration utilizing a magnetic field-applied Czochralski method (MCZ method) has been proposed as a method for inhibiting the above-described fluctuation of resistivity.
However, the use of silicon wafer made of the CZ crystal produced by the MCZ method and having low interstitial oxygen concentration includes problems such as an increase of production cost because of the application of the magnetic field and deterioration of mechanical strength of the silicon wafer after the device heat treatment because of the low interstitial oxygen concentration. In addition, since the device heat treatment of the silicon wafer having low interstitial oxygen concentration generates only a low density of oxygen precipitation induced defects, sufficient gettering ability could not be achieved in some cases.
The following patent references 1 through 3 describe techniques for solving the above-described problems.
Japanese Unexamined Patent Application, First Publication No. 2002-100631 (Patent Reference 1) describes a technique comprising growing a silicon single crystal ingot having a primary interstitial oxygen concentration of 10 to 25 ppma (JEIDA: Japanese Electronic Industry Development Association) [7.9×1017 to 19.8×1017 atoms/cm3 (Old-ASTM)] such that the silicon single crystal has a resistivity of 100 Ωcm or more; working tile silicon single crystal ingot into silicon wafers; and performing heat treatment of the wafers.
Japanese Unexamined Patent Application first Publication No. 2002-100632 (Patent Reference 2) describes a technique comprising: growing a silicon single crystal ingot by the CZ method such that the silicon single crystal ingot has a resistivity of 100 Ωcm or more and primary interstitial oxygen concentration of 10 to 25 ppma [7.9×1017 to 19.8×1017 atoms/cm3 (Old-ASTM)], and is doped with nitrogen; working the silicon single crystal ingot into wafers; and performing a heat treatment of the wafers, thereby controlling a residual interstitial oxygen concentration of the silicon wafers to be 8 ppma or less (JEIDA: Japanese Electronic Industry Development Association) [6.32×1017 atoms/cm3 or less (Old-ASTM)].
PCT International Publication for Patent Application No. 00/55397 (Patent Reference 3) describes a technique comprising: growing a silicon single crystal ingot by the CZ method such that the silicon single crystal ingot and has a resistivity of 100 Ωcm or more and primary interstitial oxygen concentration of 10 to 25 ppma [7.9×1017 to 19.8×1017 atoms/cm3 (Old-ASTM)]; working the silicon single crystal ingot into wafers; and performing oxygen precipitation heat treatment of the wafers, thereby controlling a residual interstitial oxygen concentration of the silicon wafers to be 8 ppma or less [6.32×1017 atoms/cm3 or less (Old-ASTM)].
According to the above-described techniques of Patent References 1 to 3, it is possible to depress the production cost by the use of a general CZ crystal having a high interstitial oxygen concentration and reduce the residual oxygen concentration of the wafer by the heat treatment of the wafer. Since the wafer has low residual interstitial oxygen concentration, it is possible to effectively depress the generation of oxygen donors during the device heat treatment process. In addition, it is possible to generate oxygen precipitation induced defects of high density in the bulk region by performing the oxygen precipitation heat treatment in order to reduce the residual interstitial oxygen concentration. The oxygen precipitation induced defects act as a gettering sink of heavy metals. Therefore, it is possible to expect the enhancement of the gettering ability.
However, in the techniques described in Patent References 1 to 3 using a silicon wafer having high interstitial oxygen concentration and high resistivity, it is necessary to perform a heat treatment of the silicon wafer at high temperature for a long time so as to generate oxygen precipitation induced defects of a high density and sufficiently reduce the residual interstitial oxygen concentration of the silicon wafers. Therefore, the following problems cannot be avoided.
Firstly, in the techniques described in Patent References 1 to 3, since the residual interstitial oxygen concentration is extremely reduced by the generation of an excessive amount of oxygen precipitation induced defects, there is a problem that the silicon wafer has low mechanical strength. The reduction of residual interstitial oxygen concentration of the silicon wafer may cause a deterioration of the mechanical strength of the silicon wafer. For example, this problem is obviously shown by the fact that slip length is reduced in accordance with increasing oxygen concentration (M. Akatsuka et al., Jpn. J. Appl. Phys., 36 (1997) L1422: non-patent reference 1) as a result of the slip dislocation occurring from the wafer support position or the like being fixed by the interstitial oxygen. In addition, it is known that the oxygen precipitation induced defect is a factor having influence on the strength and enhances the strength by inhibiting the movement of slip dislocation under conditions of lows heat and small dead weight stress. However, this reduces the strength by acting as a source of slip dislocation and tends to generate wafer-warpage or the like under conditions of high heat and large dead weight stress (K. Sueoka et al., Jpn. J. Appl. Phys., 36(1997)7095: non-patent reference 2). The heat and dead weight stress loaded on the silicon wafer during the device heat treatment process depend on the conditions of the heat treatment. In the techniques described in Patent References 1 to 3, mechanical strength of the silicon wafer is reduced to a low level.
Secondary, as described above, heat treatment at a high temperature for a long time is necessary in the techniques of Patent References 1 to 3. Therefore, there is a problem of the high production cost that accompanies the heat treatment. Although the production cost may be depressed by the use of a general CZ crystal having a high interstitial oxygen concentration, high frequency diode as the final product is expensive.
Third, the techniques of Patent Reference 1 to 3 have problems of heavy metal contamination of the silicon wafer within the heat treatment furnace during the time of heat treatment. For example, in Patent Reference 1, the duration of the heat treatment required to reduce the residual interstitial oxygen concentration is utmost 47 hours and at least 17 hours. Since the possibility of heavy metal contamination of the silicon wafer increases with increased heating time, there has been a high possibility of the silicon wafer suffering heavy metal contamination in the heat treatment furnace under the above-described heating condition with long duration.
Fourth, where a high-frequency diode having a high resistivity layer is produced using a silicon wafer described in Patent Reference 1 to 3, it is necessary to form recombination centers in the high-resistivity layer by the thermal diffusion of heavy metals such as Au and Pt into the substrate or by the introduction of irradiation defects by irradiating an electron beam. Therefore, production of the high-frequency diode costed substantial labor and a high production cost.
Under the consideration of the above-described circumstances, an object of the present invention is to provide a silicon wafer having a high resistance, being optimum for the production of a high frequency diode, having a sufficient density of oxygen precipitation induced defects needed for gettering, being able to effectively inhibit the generation of oxygen donors during the device heat treatment process, having sufficient mechanical strength, and being possible to be used as a high resistivity layer of a high frequency diode without requiring the formation of recombination centers in the high resistivity layer.
Another object of the invention is to provide a method for producing a silicon wafer, which is performed using a short heat treatment time and scarcely causing heavy metal contamination in the heat treatment furnace, and can be used for the production of the above-described silicon wafer with high quality and a low production cost.
Another object of the invention is to provide a silicon wafer which can be used for providing a high frequency diode having a high resistivity layer of sufficiently high resistivity and generating only few noise with low price.
Still another object of the present invention is to provide a silicon wafer having high resistivity and a method for producing the same which can be used for the production of a high resistivity diode with various advantages. The advantages include economical efficiency due to shortening the heat treatment of a wafer worked from a high resistance crystal grown by the CZ method, depressed generation of oxygen donors during the device production process, use of oxygen precipitation induced defects (oxygen precipitation nuclei or oxygen precipitates) as recombination centers, omission of the conventional formation process of recombination centers by thermal diffusion of heavy metals such as Au and Pt or by electron beam irradiation, use of the wafer having a high gettering ability and a high mechanical strength, controllability of lifetime, a high yield, and a low production cost.